The present invention relates to a synchronous AC motor which uses permanent-magnet excitation and is solely mains-commutated in the synchronous mode after starting.
Mains-commutated synchronous motors such as this operate in the synchronous mode at a synchronous rotation speed which is governed uniquely by the mains frequency and the number of pole pairs. Starting from rest and further acceleration to synchronism are in many cases ensured by special measures.
For example, EP 0 574 823 B1 describes an electronic starting apparatus for such a synchronous motor with a permanent-magnet rotor. Similar devices are also described in the other publications EP 0 654 890, EP 0 666 639, EP 0 682 404, DE 195 33 076, DE 195 33 344, DE 197 01 856 and EP 0 872 949. However, all these publications obviously relate only to motors of the internal-rotor type.
In comparison to synchronous motors, electronically commutated (DC) motors (so-called EC motors) are constructed very similarly, but an EC motor is supplied via special electronics with an electronically produced voltage whose frequency and phase angle are always determined as a function of the rotation speed and rotor position such that the motor develops an adequate torque at any given rotation speed. There are therefore far fewer problems associated with the starting of EC motors than synchronous motors. EC motors can therefore also use designs based on the external-rotor principle even if the external rotor in this case has a high moment of inertia counteracting the acceleration torque during starting owing to the rather high mass revolving on a relatively large radius. However, particularly in applications where the motor is intended to be operated directly from the AC mains, the commutation electronics for an EC motor are in fact particularly complex, owing to the high intermediate-circuit voltage. The switching elements (MOSFETs or IGBTs) need to have a high withstand voltage, and the complex actuation circuit increases the costs further. The intermediate-circuit capacitors which are required not only occupy a large volume, but also limit the life of the electronics. The electronics do not draw sinusoidal currents from the mains, and power-factor correction measures are therefore also often necessary. Overall, the electronics are not only costly, but are also difficult to integrate in the motor.
The external-rotor principle offers particular advantages for certain applications, in particular for fans, since an external-rotor motor is particularly compact owing to the short end windings and the fact that it is mounted in a manner which allows it to be integrated in the interior of the stator, so that a highly space-saving motor/fan unit can be formed by direct installation in a fan impeller. The fixed connection of all the rotating elements ensures accurate balancing, and thus little load on the bearing. Long life is furthermore achieved by virtue of the low bearing temperature, since the motor is located in the cooling flow of the air being conveyed. A further advantage of this type is the simpler and lower-cost winding technology for the stator.
Although this type of motor offers a large number of advantages for both radial and axial fans, no fans using a mains-commutated external-rotor synchronous motor are yet known from the prior art.
However, other specific configurations of external-rotor synchronous motors are known from the prior art.
For example, EP 0 050 456 describes a synchronous motor having an internal or external rotor, but this has an electromechanical commutator, that is to say a commutator with brushes.
DE-UM 71 03 331 describes an external-rotor reluctance motor.
EP 0 189 652 describes an external-rotor AC motor whose rotor comprises three layers, to be precise a permanent-magnet layer with a high coercivity force and low electrical conductivity, a soft-magnetic layer with high permeability, and a ferromagnetic layer with high permeability, high electrical conductivity and a specific electrical resistance. This motor is therefore actually a combination of an asynchronous squirrel-cage motor, a hysteresis motor and a synchronous motor with permanent-magnet excitation.
DE-A 14 88 370 describes an external-rotor synchronous motor with permanent-magnet excitation. However, it is evident from the description that this motor contains a squirrel cage in the rotor, so that this is a combination of an asynchronous motor and a synchronous motor with permanent-magnet excitation.
EP 0 431 178 describes a synchronous machine with an external rotor as a generator. According to the description, generators and motors admittedly have essentially the same mechanism, so that generators can in principle also be used as motors. However, this is not entirely correct, since there is no need for a generator to start on its own. This publication therefore also contains no specific references to use as a synchronous motor and the starting problems that occur in that case.
DE 33 20 805 describes a multi-pole multi-phase synchronous machine in the form of a ring motor. DE-C 926 434 describes a synchronous motor with material having high hysteresis in the secondary section, and in the form of an internal- or external-rotor motor. This is therefore specifically a hysteresis motor.
Finally, DE 2 234 987 describes a single-phase hysteresis synchronous motor with a specifically treated magnetic material.
The present invention is based on the object of providing a synchronous AC motor which combines the advantages of a pure synchronous motor with permanent-magnet excitation and mains commutation and those of an EC motor of the external-rotor type.
According to the invention, this is achieved, on the basis of a mains-commutated, synchronous AC motor with permanent-magnet excitation, by this motor being configured as an external-rotor motor with a permanent-magnet external rotor which rotates around an inner stator.
This configuration according to the invention of a mains-commutated synchronous motor with permanent-magnet excitation as an external-rotor motor was in no way obvious to a person skilled in the art. The extensive prior art referred to above makes it clear that the specialist world had in fact been prejudiced, assuming without exception that such a synchronous motor with an external rotor would not start in a way such that it would reach synchronism owing to its high moment of inertia (long mechanical time constant). Furthermore, it had been assumed that, owing to the higher moment of inertia, a considerably greater amount of energy would be necessary for starting and that a correspondingly higher motor current for a greater flux would be necessary for this purpose which, however, would also mean a greater risk of demagnetization of the rotor magnet. It therefore appeared to be improbable that acceleration could be achieved without demagnetization of the rotor magnet. Accordingly, it is surprising to find that the external-rotor synchronous motor according to the invention can be started just by using simple starting methods which are known per se, that is to say that starting and acceleration are possible without demagnetization, despite the increased moment of inertia of the external rotor. A special, novel starting apparatus with particular starting actuation of the motor is preferably used in order to reach synchronous operation reliably. In this case, according to the invention, the starting of the external rotor is subdivided into a number of phases, in each of which the stator winding is actuated in a different manner, matched in an optimum manner to the respective rotation speed. These special measures will be described in even more detail further below.
Further advantageous refinement features of the invention are contained in the dependent claims.